The present invention relates to a spring-type brake actuator for a vehicle braking system, and in particular to a spring brake actuator configured to apply a vehicle brake using tension force rather than compression force.
It is well known to employ so-called “spring brake” actuators to provide service brake, parking bake and emergency brake functions on vehicles such as commercial trucks (such as tractors, trailers, busses and box trucks) equipped with lever-operated drum or disc brakes. Spring brake actuators are typically pneumatically operated and supplied with operating air from a compressed air source on the vehicle. These actuators also typically are arranged so that their brake operating rod extends outward from the actuator to apply the brake to which they are connected, and arranged in a “fail-safe” manner, i.e., where the actuator defaults to a brake application state upon loss of operating air pressure.
An example prior art spring brake actuator is shown in cross-section view in FIG. 1. Actuator housing 1 includes a rear cylinder 2 in which a rear piston 3 is displaceably arranged. The inner wall of the rear cylinder and a chamber-side of the rear piston define a rear ventilation chamber 4. The other side of the rear piston bears on a brake actuator spring 5. This spring is also known in the art as a “power spring” or a “parking brake spring,” and these terms may be used interchangeably.
The rear ventilation chamber is isolated from the spring side of piston 3 by an annular seal 6. An intermediate flange 8 (also known as a “wall”) separates rear cylinder 2 from a front cylinder 9. The intermediate flange 8 traversed by a seal 10 through which passes a parking brake application rod 11, formed as an extension of rear piston 3. The parking brake application rod 11 can be displaced in the intermediate flange 8 by the rear piston. A front ventilation chamber 7 within front cylinder 9 is delimited by the cylinder inner wall and a front piston 13 and annular diaphragm 14. The rear piston 3 and the front piston 13 are in non-coupled contact with one another by means of the parking brake application rod 11, such that the front piston 13 can be displaced in a brake application direction by the rear piston 3 and/or by the application of pneumatic pressure in front ventilation chamber 7. An actuating rod 15 for actuating a brake lever of a vehicle brake is provided on the front side of the front piston 13.
FIG. 1 also shows mounting studs 16 provided for mounting of the actuator 1 on the vehicle brake, as well as a light return spring 18 which biases front piston 13 toward the rear of front chamber 7.
When no pneumatic pressure is present in the FIG. 1 actuator unit, the brake actuation spring 5 applies a high spring force to rear piston 3, which in turn applies this force via parking brake application rod 11 to front piston 13 to cause the actuator rod 15 to apply the vehicle brake. In this state, the vehicle brake functions as a parking brake, preventing vehicle movement.
When release of the parking brake is desired, the rear ventilation chamber 4 is filled with compressed air via a ventilation port (not illustrated). As the force generated by the increasing air pressure on the front side of rear piston 3 exceeds the force generated by brake application spring 5 (aka “power spring”), the rear piston 3 and parking brake application rod 11 move toward the rear of the rear cylinder 2, compressing spring 5 and causing air in the rear of rear cylinder 2 to be vented to atmosphere through passages in rear piston 3 (not illustrated) to vent path 19.
As parking brake application rod 11 moves towards the rear, the force previously applied to front piston 13 is relieved, and the return spring 18 biases the front piston 13 toward the rear of front cylinder 9, thereby withdrawing actuating rod 15 away from and releasing the vehicle brake. The vehicle therefore moves from a state in which it is braked by the brake actuator spring 5, to a non-braked state in which the vehicle may be moved. The vehicle brake is applied as a service during normal operation by admitting compressed air into the front ventilation chamber 7 (via a port not shown in FIG. 1). Because air pressure in rear ventilation chamber 4 continues to hold parking brake application rod 11 at the rear of the rear cylinder 2, the front piston 13 and actuating rod 15 are free to move forward and backward within the front cylinder as necessary to respond to the operator's brake actuation demands.
As can be seen for example in FIG. 1, the prior art push-type spring brake actuators typically have a relatively large portion of their mass suspended a considerable distance from the brake caliper mounting flange. The location of the center of gravity (CG) of the actuator creates a large moment about the mounting flange, requiring a robust structure at the brake-end of the actuator in order to ensure the actuator can withstand the vibrations and other forces applied to the actuator over its life, including being robust enough to avoid structural failure from fatigue. The additional material required to form an adequately robust actuator also increases costs and the space required by the actuator, a scarce resource in the cramped region around the end of an axle, particularly a steering axle.
The present invention addresses these and other problems in the prior art with a spring brake actuator that minimizes installation space requirements, materials use and costs with a pull-type double diaphragm spring brake actuator. This invention further requires little or no modification of existing lever-actuated brake designs. For example, the inventive actuator may be used with an existing drum brake by merely reversing the brake's so-called “S-cam” brake shoe actuator so that the brake's operating lever that rotates the S-cam can be used in a pull-type vs. push-type manner. Alternatively, the inventive pull-type actuator may be mounted on the opposite side of an existing brake's operating lever in a pull-type arrangement without the need for any modifications to the brake itself.
In one embodiment of the present invention the center of mass of the pull-type spring brake actuator is positioned closer to its mounting flange, in part due to the location of the relatively heavy power spring immediately adjacent to the mounting flange. In this embodiment the parking brake piston is located closer to the brake lever end of the actuator than the service brake piston, and the parking brake is applied by moving the parking brake piston away from the brake lever under the influence of the expanding power spring, pulling the brake application operating rod with the piston. The parking brake is released by application of fluid pressure (such as pneumatic pressure) to a chamber on a side of the parking brake piston opposite the power spring side of the piston.
Once the parking brake is released, a separate chamber containing the service brake piston located on the side of the parking brake chamber opposite the actuator mounting flange may be pressurized to pull the brake application rod into the actuator to applied the brake during normal vehicle operation. The operating rod is provided with a flange member in the portion of the rod in the parking brake chamber which cooperates with the parking brake piston to allow the operating rod to pass through parking brake piston when the parking brake is released, and to be engaged by the parking brake piston when the power spring biases the parking brake piston in the brake application direction.
The arrangements of the present invention permit the use of lighter, less costly components. For example, the brake application operating rod may have a smaller diameter (potentially as small as ¼″) because it is only loaded in tension, eliminating compressive buckling concerns. Similarly, the parking brake portion of the actuator housing may be made thinner than in a push-type spring brake actuator because the end wall portion will be supported by the actuator mounting flange to which the actuator is bolted. Further, because the mounting studs are adjacent to the power spring and the end wall at the mounting flange, reinforcement of the end wall in the region of the mounting studs may be unnecessary. In addition, an intermediate flange between the parking brake chamber and the service brake chamber may be made lighter, as at least the outer portion of this flange no longer needs to be strong enough to withstand the forces applied in the prior push-type spring brake actuators by the power spring and the parking brake release pressure.
In comparison to an example of an existing push-type spring brake actuator, an EverSure® T30/30-3″ spring brake actuator available from Bendix Spicer Foundation Brake LLC, Elyria, Ohio, the inventive pull-type spring brake actuator provides a greater than 10% reduction in actuator weight (16.7 lb. vs. 19 lb.), a greater than 25% improvement in reduction of distance of the center of mass from the mounting flange (3.9″ vs. 5.4″), a greater than 10% reduction in the number of parts (31 vs. 36), and a substantial reduction in both material and manufacturing costs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.